This invention relates to a radiant wall oven of the type used for curing painted vehicle bodies. Such an oven includes a heating chamber for the vehicle body which is separated from a hot air chamber by a radiant wall. Air is circulated between the hot air chamber and a separate air heating chamber to heat the radiant wall.
An example of a prior art radiant wall oven is shown in FIG. 1. Oven 10 comprises a heating chamber 11 separated from a hot air chamber 12 by a radiant wall 13. Air in an air heating chamber 14 is heated by a heat source 15 such as a burner and is circulated by a blower 16 from air heating chamber 14 through a supply duct 17 to hot air chamber 12 and back through return duct 18 to air heating chamber 14 for reheating. Hot air in hot air chamber 12 heats radiant wall 13 to a high temperature; and radiant wall 13 radiates heat in heating chamber 11 to heat a heating load such as a painted vehicle body.
The temperature of radiant wall 12 is controlled by a standard PID (proportional, integral, derivative) closed loop control 20 which compares a sensed control temperature signal to a predetermined setpoint value. In the prior art, the sensed control temperature signal has been obtained from a temperature sensor 21 (signal T.sub.1) on the radiant wall itself or a temperature sensor 22 (signal T.sub.3) in return duct 18. However, use of either of these signals in a single closed loop control is subject to time delay required for changing the temperature of the air in air heating chamber 14, conveying the new temperature air through supply duct 17 to hot air chamber 12 and heating or cooling radiant wall 13.
Such a control system is capable of maintaining a reasonably constant temperature under stable conditions; but it requires time, due to the time delays already discussed, to recover from a change in heating load in the oven. The introduction of a new vehicle body to heating chamber 11 presents a significant change in heating load to the oven and thus initiates a transition period before the radiant wall temperature is again stabilized at the desired temperature. During this transition period, the paint on the vehicle body is not exposed to the desired temperature; and the length of the transition period should be minimized for the best process control. However, attempts to decrease this transition period by increasing closed loop gain may lead to overshoot and instability in the control. The production of vehicles has become a highly competitive business in which finished surface quality is one of the features by which vehicles are judged and in which, therefore, the paint application process must be finely controlled. Due to the same competitive situation, however, there is incentive to reduce production time and thus decrease costs.